The invention concerns a method for the measurement of nuclear magnetic resonance in selected regions of a body for the purpose of imaging body cross sections with which the body is exposed to both a homogeneous magnetic field as well as to a selection gradient and is excited with a selective excitation impulse and, subsequently, the selection gradient is replaced by time-limited phase encoding and read gradients which are perpendicular to the selection gradient and to each other and the body is irradiated with a sequence of refocussing pulses through which a plurality of nuclear magnetic resonance signals are excited in the form of so-called spin echoes, whereby the switching-in time and strength of the gradients is adjusted to the pulse sequence in such a fashion that, at the time of occurrence of each refocussing pulse relative to the effect of the gradient, the nuclear spins have the same phase state as that at the point of time of the preceding refocussing pulse, whereby the read gradient is switched-in during the occurrence of the spin echo to be symmetric to same and the phase encoding gradients are switched-in following each refocussing pulse and are terminated before the appearance of the spin echo, switched-in again, with opposite directional influence, after the appearance of the spin echo and terminated once more before the next refocussing pulse, whereby the strength and/or duration of the phase encoding gradient is changed after every refocussing pulse and the spin echoes are computed into imaging signals taking into consideration the change in the phase encoding gradient in accordance with the two-dimensional Fourier transformation method.
This so-called RARE method is known in the art from U.S. Pat. No. 4,818,940.
The RARE method is a rapid imaging method with which spin echoes are produced following one excitation pulse through a plurality of refocussing pulses which are differently phase encoded through the application of suitable magnetic field gradients. In this fashion the number of differing phase encoding steps is significantly greater than the number of excitation pulses. In extreme cases all phase encoding steps which are necessary for image reconstruction can be obtained following one single excitation through production of correspondingly long echo trains. Due to the behaviour of transverse magnetization under the influence of such a sequence of equally spaced refocussing pulses, the sequence is extremely stable with regard to flow, motion, diffusion and magnetic field inhomogeneties which can occur in biological tissue due to, for example, changes in susceptibility.
A number of magnetic resonance imaging applications are precisely intended to measure or image these above mentioned effects. The purpose of the present invention is therefore to modify the known RARE method sequence in such a fashion that RARE is sensitive to such effects while maintaining the otherwise advantageous properties of rapid imaging and high stability of the Carr-Purcell-Meiboom-Gill-echo train.